At today’s close, RDS-A has an 8% premium to RDS-B, which makes absolutely no sense insofar as the two classes of shares have the same economic interest (as you noted). This discrepancy must be the result sign of a poorly functioning market for ADSs due to the severe volatility in securities markets generally.
“The efficient-market hypothesis may be the foremost piece of B.S. ever promulgated in any area of human knowledge!”